A voltmeter is an instrument used for measuring the electrical potential difference between two points in an electrical circuit.
Voltmeters are made in a wide range of styles. Instruments permanently mounted in a panel are used to monitor generators or other fixed apparatus. Portable instruments, usually equipped to also measure current and resistance in the form of a multimeter, are standard test instruments used in electrical and electronics work.
General purpose analog voltmeters may have an accuracy of a few percent of full scale, and are used with voltages from a fraction of a volt to several thousand volts. Digital meters can be made with high accuracy, typically better than 1%. Specially calibrated test instruments have higher accuracies, with laboratory instruments capable of measuring to accuracies of a few parts per million. Meters using amplifiers can measure tiny voltages of microvolts or less.
Root mean square (RMS) voltmeters are commonly used to measure AC voltages and the equivalent power of complex waveforms. Currently, RMS voltmeters have limited bandwidth and can become very complex, expensive, and large. Many RMS voltmeters currently use semiconductor diode networks to form an approximation of the absolute value of the incoming voltage. However, they are not entirely successful at doing this because of the non-zero voltage at which semiconductor diodes switch.